Mean surface (nondissected)
Major components of luminal content
Water content Dry matter content Bacteria Major ions
SCFA's anions: acetate, propionate, butyrate, etc.
Major physiological functions
Fermentation of nondigestible food components Transport of Na+/Cl via Na+/H+ and Cl/HCO3
exchanges Water absorption Stool formation fulfill. There is, however, a high inter-individual variation in the relative percentage of species (e.g., 109 to 1013 cfu/g for bacteroides and 105 to 1013 cfu/g for bifidobacteria4). The nondigested part of the chyme that leaves the small intestine reaches the large bowel and affects ecological, physiological, and metabolic events. Inside the colon, the transit is slow and the time of residence is long (±54 h).11 Consequently, bacteria (mostly if their number is 107/g) have time to breakdown and to ferment a large proportion of the nondigested components of the chyme as well as endogenous compounds (mainly proteins and carbohydrates). Fermentation allows bacteria to grow and proliferate, but it also salvages part of the energy of the nondigestible nutrients and produces gases (H2, CH4, CO2, H2S), short chain car-boxylic acids (mainly acetic, propionic, butyric acids), better known as the short chain fatty acids (SCFAs), and other miscellaneous compounds (ammonia, amines, phenols).11112 The colon behaves, thus, like a fermenter that feeds the bacteria and provides the host with physiologically active, but sometimes also potentially toxic, metabolites. A total amount of 50-70 g/d of nondigestible dietary substrates (Table 5.3) is needed to maintain the colonic microflora1.
The activities of the colonic bacteria are affected by the physiology and the anatomical architecture of their environment, and they fluctuate in response to substrate availability and distribution in the different segments of the large bowel, to luminal pH, to redox potential (Eh), and to oxygen concentration (pO2).1513 In the right/proximal colon (cecum and ascending segment), dietary nutrients are abundant; bacteria grow fast and produce large quantities of SCFAs that acidify the
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